This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Mycobacterium Tuberculosis (MBT) Protease is a 14kD protein that plays a role in the defense of the bacterium against eradication by the host. The protease is known from X-ray crystallography to have an interesting annular structure, composed of alternating alpha and beta rings with a number of non-functional cysteines that are candidates for spin labeling. One model of protease function is that the rings rotate with respect to one another leading to the decomposition of proteins that enter the annular region. As a first step to check this hypothesis, spin-labeling experiments have been performed on singly-labeled MBT protease, where all but one of the non-functional cysteines have been mutated to a residue without a thiol group, such that only one spin label per protease is possible. Initial results at X-band indicate that the protocol for spin-labeling the MBT protease needs to be improved, as the signal to noise ratio was low. Consultations with ACERT staff have led to improved sample preparation protocols. The eventual goal is to perform distance measurements on bi-labeled MBT protease, to resolve the distribution of distances corresponding to different relative orientations of the alpha and beta rings.